Regional winners (3), Chairman’s winners (1), and Rookie All-Star winners (1) are the qualifications at regional level. Then you have the tiers. It is possible for a team to have more than one slot reserved due to tier, then winning something at a regional. Others may get slots from a regional IF it’s a special situation. It’s happened only once, so don’t count on it happening.
Also the Engineering Inspiration award winner from each regional. That’s 6 per regional, a total of 198. FIRST is reserving 100 spots at the Championship for regional qualifiers (340 teams, minus 240 open registration). That means they assume half of the qualifying teams will have already open-enrolled. Although I suspect the 340 team figure isn’t set in stone, they could expand by a few if needed.
It should be noted that Rookies can qualify for the Championship ONLY by being a Regional Winner or Regional Rookie All-Star.
FIRST wants to have a full complement of teams at the Championship, so they do correct for teams that double qualify, as well as those who qualify, but are unable to attend. (cost, school travel policy, etc.)